This invention relates to an apertured color selection electrode or mask for use in a color cathode ray tube, and more particularly relates to such a mask which is held under mechanical tension.
A common type of color cathode ray tube (CRT) used in color television and allied color display applications such as computers, oscilloscopes, etc., employs an apertured color selection electrode or mask to control passage of the electron beams to the proper locations on the cathodoluminescent display screen.
In the case of television, the CRT employs three electron beams, one for each of the primary color (red, blue and green) components of the color video signal, and employs a screen made up of an array of phosphor elements luminescing in the three primary colors. The apertured mask is located a short distance behind the screen to intercept the electron beams, and has a large number of apertures located to allow passage of each beam to the phosphor elements of the corresponding color.
The mask is fabricated from a relatively thin sheet of metal such as steel, and is thus susceptible to thermal expansion when heated, primarily by impingement of the electron beams. Such expansion moves the mask closer to the screen, which can change the registration of the apertures with the phosphor elements. During an initial warm-up period, the various tube components will expand at various rates, but will eventually come to an approximate state of thermal equilibrium, at which the tube is designed to operate. However, during normal operation, transient heating in localized areas of the mask occurs when the beam intensity is high, for example, to portray highlights in the display on the screen. This localized heating causes a transient localized expansion of the mask known as "doming". This doming can cause mis-registration between the apertures and the phosphor elements, which degrades the color purity of the display.
Various techniques have been employed in an attempt to minimize doming. These include reducing the heating or increasing the cooling of the mask, such as by coating the back of the mask with a material having a high electron back scattering coefficient, to reduce heating of the mask by the electron beams, or by coating the back of the screen with a material having a high thermal emissivity, to conduct heat away from the mask. However, these techniques introduce new materials and add extra steps to the manufacturing process, and tend to decrease luminance and/or contrast of the display.
Another technique is to fabricate the mask from a material having a relatively low thermal expansion, such as an iron-nickel alloy containing about 36 weight percent nickel, balance mostly iron, known commercial by the name Invar. While Invar masks exhibit less doming than conventional steel masks, they are more expensive, due both to higher material cost and to lower yields. More effective in reducing doming is to place the mask under mechanical tension.
Two examples of tension masks in current production are the Sony Trinitron and the Zenith FTM (flat tension mask) tubes. The FTM tube employs a so-called dot screen, in which the phosphor elements are in the form of triads of red, blue and green dots, requiring registration with and tension in both the longitudinal and transverse directions of the mask. The Sony tube uses a more conventional striped screen, in which the phosphor elements are in the form of longitudinally-oriented triads of red, blue and green stripes, and thus requires registration only in the transverse direction.
The Sony mask is a grid structure of grid elements stretched longitudinally over a substantially rectangular, one-piece rigid frame. The grid elements are stretched between the supports of the frame by an amount sufficient that they will remain taut even during heating and expanding. This is accomplished by loading to effect resilient bending of the sides of the frame, securing the grid elements to the top and bottom of the frame, and removing the load, allowing the sides to return to their original positions, thereby causing the desired longitudinal stretching of the grid elements.
Exemplary structures in which the required resilience in the sides of the frame is achieved by the use of resilient U-shaped side supports and by cutting recesses into the sides of the frame are described in U.S. Pat. Nos. 3,638,063 and 4,333,034, respectively. A variation of the latter design in which the recesses are replaced with leaf springs is described in U.S. Pat. No. 5,214,349.
With such structures, changes in tension of the grid elements caused by thermal expansion are compensated for by the shrinkage of the grid elements or by a slight restoring force of the frame (see U.S. Pat. No. 3,638,063, col. 4, lines 18-22).
In a further variation on this theme, described in JP-A 5-114356, both the side supports and the top and bottom members are deformed during assembly, and thereafter provide a restoring force to maintain the grid elements in tension.
Unfortunately, in order to maintain the grid elements in a high degree of tension, such grid structures tend to be relatively heavy and rigid, and require relatively complex and expensive manufacturing techniques to produce. In addition, such rigid structures are less efficient in reducing localized doming than in reducing overall doming.